The subject matter disclosed herein relates generally to magnetic resonance imaging (MRI) systems, and more particularly to methods and systems for correcting a B0 field in MRI imaging using shim coils.
MRI is a medical imaging modality that generates images of the inside of a human body without using x-rays or other ionizing radiation. MRI or Nuclear Magnetic Resonance (NMR) imaging generally provides for the spatial discrimination of resonant interactions between Radio Frequency (RF) waves and nuclei in a magnetic field. Specifically, MRI utilizes hydrogen nuclear spins of the water molecules in the human body, which are polarized by a strong, static magnetic field of a magnet. This magnetic field is commonly referred to as B0, or the main magnetic field. When a substance, such as human tissue, is subjected to the main magnetic field, the individual magnetic moments of the spins in the tissue attempt to align with the main magnetic field. The magnetic moments that are associated with the spins become preferentially aligned along the direction of the magnetic field, resulting in a small net tissue magnetization along an axis of the magnetic field. When excited by an RF wave, the spins precess about the main magnetic field at resonance frequency of the hydrogen nuclei, commonly referred to as the Larmor frequency.
The MRI system also includes a superconducting magnet that generates the main magnetic field within an imaging volume. The main magnetic field is essentially a large field with small non-homogeneous characteristics in select portions of the field. Manufacturing processes, as well as equipment and site conditions, create the non-homogeneous characteristics in the main magnetic field B0. In operation, the non-homogeneous characteristics in the magnetic field B0 may distort the position information in the imaging volume and degrade the image quality.
The MRI system uses various types of RF coils to create pulses of RF energy at or near the Larmor frequency. The RF coils transmit RF excitation signals and receive magnetic resonance (MR) signals that the MRI system processes to form the images. Traditional RF coils have discrete capacitive elements at select points about a circumference of a loop to tune the RF coil to a select resonance frequency to receive the RF energy.
Inductive elements are located in parallel with each capacitive element to permit correcting current to flow through the RF coil. The inductive elements are large to limit heating during RF signal transmission and reception. The inductive elements are also resistive and lead to electrical losses. Further, the inductive elements limit the amount of corrective current that may be applied. For example, a large direct current in in the inductive element may induce a magnetic field that may interfere with the main magnetic field.